Prior art ceramic bandpass filters, such as that shown in FIG. 1 and described in U.S. Pat. No. 4,431,977, are typically constructed from blocks of ceramic material. The blocks are typically formed by pressing a ceramic-based powder, using a mold or other equivalent, to form a solid structure. The resulting structure may then be cured, or fired, to form a rigid block of ceramic. The block, including any number of through holes (e.g., holes 140 shown in FIG. 1) which make up the individual resonator structures, is then selectively coated with a conductive metallization layer. The coating is typically applied to the block so as to provide a shorted, typically one-quarter wavelength, transmission line resonator with each of the holes. Further processing of the metallization, as next described, is required to tune the resonator/filter to the desired frequency characteristics.
FIG. 2 shows a top view of a prior art ceramic block filter having an intricate metallization pattern on the top surface. The filter 200 is described in U.S. Pat. No. 4,692,726 (issued to Green et al. on Sep. 8, 1987, and assigned to the assignee of the present invention). The metallization pattern on the top surface of a dielectric filter is commonly known to affect the capacitive loading on the top surface of the dielectric filter. The pattern may be made up of a ground plane coating (203), input/output pads (201), and various resonator pads (202, 204) which surround the hole resonators. By changing the thickness, area, and relative spacing among these metallized areas, the capacitive loading at the top of the block can be altered. Altering the capacitive top loading is a well-known method for frequency tuning dielectric resonators and filters, as the capacitive reactance plays a significant role in the overall frequency response characteristics (i.e. center frequency, bandwidth, etc.).
Detailed metallization patterns, like the one shown in FIG. 2 are typically screen printed, e.g., using a plating mask or similar article, onto the top surface of the block. The results of this process have proven to be greatly dependent on the registration of the block with respect to the plating mask. That is, even a slight mis-alignment between the mask and the block often results in a resonator which is either unusable, or one that needs a substantial amount of tuning to meet the required specifications. Most tuning techniques today involve removing portions of the metallized top-patterns, which operations are often manual (e.g., using a hand-held grinding tool). That is, wide process variations seen during the manufacture of such dielectric resonators (e.g., forming the block, deposition of the metallization patterns, and the manual tuning process) sum together to produce a resonator or filter whose electrical characteristics are widely variable. As in any other manufacturing process, wide process variation leads to reduced overall yields (i.e., number of products which meet the specifications and can be shipped), and increased manufacturing costs.
Accordingly, a need exists for a ceramic block resonator or filter, and method for electrically tuning such a resonator or filter, which is not constrained by the aforementioned shortcomings. In particular, where a ceramic block filter or resonator requires frequency tuning to tight tolerance, an improved apparatus and cost effective method for providing such tuning, would be an improvement over the prior art.